The integrated activity recorded from the red nucleus in unrestrained, unanaesthetized cats increases phasically during the outbursts of rapid eye movements which are characteristic of the desynchronized phase of sleep. Bilateral destruction of the vestibular complex, particularly when localized to the medial and descending vestibular nuclei, abolishes not only the rapid eye movements but also the related phasic increases of rubral activity. These results suggest that ascending volleys originating from the vestibular nuclei during desynchronized sleep excite not only the neurones of the oculomotor nuclei but also those of the red nucleus.